Ideally to ensure the well being of your ponies you need to have good, weed and ragwort free grass. Ideally horses also like clover and mixed herbs in the grass but if possible not buttercups.
A cheap way to clear most weeds is to use a product called Agretox 50 to spray the land with a sprayer on a tractor or to use Grazon 90 if using a back pack sprayer. Well managed grass land can produce much more good grass eating than badly managed land. We have found that the regular use of 25.5.5. Fertilizer applied twice during the growing season works miracles.
It is important to make sure that the land has a pH of 5.6 to 5.9 as grass grows better at this level. We are in an acid soil area and so have to lime our land every six years with 3 tons per acre to keep the pH right. This costs approximately £25 per ton of lime including the spreading.
It is also best to get your soil tested for mineral deficiency; we did this and found that our soil was lacking magnesium which horses need. We have solved this problem by the use of a relatively cheap product called Grass track which costs about £20 a bag which is enough for an acre of land per year.
The regular use of a chain harrow or tine harrow are recommenced as they bruise the grass and loosen the surface of the soil making the grass get more dense which then helps to suppress the weeds. If weeds are a major problem in midsummer then it is best to cut them using a topper.
If you only have a few acres of land then a local farmer will usually perform these tasks for you for an appropriate fee which is usually much less than you think.